Here's why:
* Exodia the Forbidden One wins the game automatically when all five pieces are in your hand, regardless of whether Exodia Necross is on the field.
* Exodia Necross is a powerful monster in its own right, but it doesn't directly affect the Exodia win condition.
Why might someone choose to summon Exodia Necross?
* Additional Attack Power: Exodia Necross is a strong monster with 3000 ATK, providing a significant offensive threat.
* Preventing Removal: Sometimes, your opponent might try to destroy your Exodia pieces. Exodia Necross acts as a powerful deterrent, making them hesitant to target your hand.
* Flexibility: Exodia Necross can be a useful card even if you don't win with Exodia, as it can act as a powerful boss monster.
In short, summoning Exodia Necross is not mandatory to win with Exodia, but it can be a strategic choice in certain situations.